From the viewpoints of environmental load reduction and cost reduction in recent years, reduction in usage of steel sheets used for food cans and beverage cans has been required, so that thickness reduction of a steel sheet has been advanced regardless of a two-piece can or a three-piece can. Associated with this, deformation of a can body due to external forces applied in the handling in can production and conveying steps and a market and buckling deformation of a can body portion due to fluctuation of the pressure in the inside of a can in heat sterilization of contents have been regarded as problems.
Conventionally, the strength of the steel sheet has been enhanced to improve the buckling deformation resistance of the can body portion. However, when the strength (YP) is increased by enhancing the strength of the steel sheet, the formability is degraded and a problem occurs in the can production step. That is, the formability is usually degraded by enhancing the strength of the steel sheet. As a result, there are problems that the incident of neck wrinkles and flange cracks increases in neck forming and the following flange forming performed after forming can body portion and a problem that an “ear” becomes large in drawing of a tow-piece can because of the anisotropy of the material. As described above, enhancement of the strength of the steel sheet is not always appropriate as a method for compensating degradation of the buckling deformation resistance associated with thickness reduction of the steel sheet.
On the other hand, the buckling phenomenon of the can body portion occurs due to degradation of the rigidity of the can body because of thickness reduction of the can body portion. Therefore, in order to improve the buckling deformation resistance, a method is considered, in which the rigidity is improved by increasing the Young's modulus of the steel sheet in itself. In particular, as for the tow-piece can, the circumferential direction of the can body after forming does not become a specific direction of the steel sheet and, therefore, the Young's modulus has to be improved uniformly in the steel sheet plane.
There is a strong interrelation between the Young's modulus of iron and the orientation. An orientation group (α-fiber) having the <110> direction, which is developed by rolling, parallel to the rolling direction particularly increases the Young's modulus in the direction at 90° to the rolling direction, and an orientation group (γ-fiber) having the <111> direction parallel to the direction of the normal to the sheet surface can increase the Young's moduli in the directions at 0°, 45°, and 90° to the rolling direction up to about 220 GPa. On the other hand, when the orientation of the steel sheet does not show alignment in a specific orientation, that is, the texture is random, the Young's modulus of the steel sheet is about 205 GPa.
For example, Patent Literature 1 discloses a steel sheet for a high-rigidity container, which is a rolled steel sheet containing, on a weight percent basis, C: 0.0020% or less, P: 0.05% or less, S: 0.008% or less, Al: 0.005% to 0.1%, N: 0.004% or less, 0.1% to 0.5% of at least one of Cr, Ni, Cu, Mo, Mn, and Si in total, and the balance being Fe and incidental impurities, which exhibits a microstructure having a ratio of a major axis to a minor axis of a crystal grain of 4 or more, and which has a maximum modulus of elasticity of 230,000 MPa or more. Furthermore, a method for enhancing the rigidity of the steel sheet is disclosed, wherein after a steel having the above-described chemical composition is cold rolled and is annealed, a strong rolling texture is formed by performing secondary cold rolling at a rolling reduction of 50% or more to increase the Young's modulus in the direction at 90° to the rolling direction.
Patent Literature 2 discloses a method for manufacturing a steel sheet for a container, wherein a steel containing, on a weight percent basis, C: 0.0020% or less, Mn: 0.5% or less, P: 0.02% or less, S: 0.008% or less, Al: 0.005% to 0.1%, N: 0.004% or less, and the balance being Fe and incidental impurities is subjected to common hot rolling and pickling, cold rolling at a rolling reduction of 60% or more is performed and, thereafter, annealing is not performed at all.
Patent Literature 3 discloses a method for manufacturing a steel sheet for a container, wherein a steel containing, on a weight ratio basis, C: 0.003% or less, Si: 0.1% or less, Mn: 0.4% or less, S: 0.015% or less, P: 0.02% or less, Al: 0.01% to 0.1%, N: 0.005% or less, and the balance being Fe and incidental impurities is hot rolled at a temperature of the Ar3 transformation temperature or less under at least a total rolling reduction of 50% or more, pickling and cold rolling at 50% or more are performed and, thereafter, annealing is performed at 400° C. or higher and a recrystallization temperature or lower. A method for increasing the value of the maximum modulus of elasticity in the plane is disclosed, wherein a rolling texture is formed in accordance with an increase in the rolling reduction of cold rolling. In this regard, the recrystallization temperature here is defined as a temperature at which the degree of recrystallization becomes 10%, where a change in the texture associated with proceeding of the recrystallization is hardly observed.
Patent Literature 4 discloses a steel sheet for a high strength can, containing, on a percent by weight basis, C: 0.003% or less, Si: 0.02% or less, Mn: 0.05% to 0.60%, P: 0.02% or less, S: 0.02% or less, Al: 0.01% to 0.10%, N: 0.0010% to 0.0050%, Nb: 0.001% to 0.05%, B: 0.0005% to 0.002%, and the balance being Fe and incidental impurities, wherein in the sheet thickness center portion, (accumulation intensity of {112}<110> orientation/accumulation intensity of ({111}<112> orientation) 1.0 is held, the tensile strength in the direction at 90° to the rolling direction is 550 to 800 MPa, and the Young's modulus in the direction at 90° to the rolling direction is 230 GPa or more.